You can make the latest version of the grammar with command yacc -v parser.y.
Here is the grammar for the packetdrill scripting language, as of the time of
writing (October 2019), produced with bison -v parser.y:
$accept: script $end
script: opt_options opt_init_command events opt_cleanup_command
opt_options: %empty
| options
options: option
| options option
option: option_flag '=' option_value
| option_flag
option_flag: OPTION
option_value: INTEGER
| WORD
| STRING
| IPV4_ADDR
| IPV6_ADDR
| IPV4
| IPV6
| WORD '=' WORD
| WORD '=' STRING
| WORD '=' BACK_QUOTED
opt_init_command: %empty
| init_command
init_command: command_spec
events: event
| events event
event: event_time action
event_time: '+' time
| time
| '*'
| time '~' time
| '+' time '~' '+' time
time: FLOAT
| INTEGER
action: packet_spec
| syscall_spec
| command_spec
| code_spec
packet_spec: tcp_packet_spec
| udp_packet_spec
| icmp_packet_spec
tcp_packet_spec: packet_prefix opt_ip_info opt_port_info flags seq opt_ack opt_window opt_urg_ptr opt_tcp_options
udp_packet_spec: packet_prefix opt_ip_info UDP opt_port_info '(' INTEGER ')'
icmp_packet_spec: packet_prefix opt_ip_info ICMP icmp_type opt_icmp_code opt_icmp_mtu opt_icmp_echo_id opt_icmp_echoed
packet_prefix: direction
| packet_prefix IPV4 opt_ip_info IPV4_ADDR '>' IPV4_ADDR ':'
| packet_prefix IPV6 opt_ip_info IPV6_ADDR '>' IPV6_ADDR ':'
| packet_prefix GRE ':'
| packet_prefix GRE opt_comma gre_header_expression ':'
| packet_prefix MPLS mpls_stack ':'
gre_header_expression: gre_flags_list opt_comma gre_sum opt_comma gre_off opt_comma gre_key opt_comma gre_seq
gre_flags_list: FLAGS '[' gre_flags ']'
| FLAGS any_int
gre_flags: gre_flag
| gre_flag ',' gre_flags
gre_flag: NONE
| CHECKSUM PRESENT
| KEY PRESENT
| SEQUENCE PRESENT
gre_sum: SUM any_int
gre_off: OFF any_int
gre_key: KEY opt_equals any_int
gre_seq: SEQ any_int
opt_comma: %empty
| ','
opt_equals: %empty
| '='
mpls_stack: %empty
| mpls_stack mpls_stack_entry
mpls_stack_entry: '(' LABEL INTEGER ',' TC INTEGER ',' opt_mpls_stack_bottom TTL INTEGER ')'
opt_mpls_stack_bottom: %empty
| '[' WORD ']' ','
icmp_type: WORD
opt_icmp_code: %empty
| WORD
opt_icmp_echoed: %empty
| '[' UDP '(' INTEGER ')' ']'
| '[' seq ']'
| '[' RAW '(' INTEGER ')' ']'
opt_icmp_mtu: %empty
| MTU INTEGER
opt_icmp_echo_id: %empty
| ID INTEGER
opt_port_info: %empty
| INTEGER '>' INTEGER
direction: '<'
| '>'
tos_spec: ip_ecn
| ECT01
| TOS HEX_INTEGER
ip_ecn: NO_ECN
| ECT0
| ECT1
| CE
flags: WORD
| '.'
| WORD '.'
| '-'
flow_label: FLOWLABEL HEX_INTEGER
ip_info: tos_spec
| flow_label
| TTL INTEGER
| tos_spec ',' flow_label
opt_ip_info: %empty
| '(' ip_info ')'
| '[' ip_info ']'
seq: INTEGER ':' INTEGER '(' INTEGER ')'
opt_ack: %empty
| ACK INTEGER
opt_window: %empty
| WIN INTEGER
opt_urg_ptr: %empty
| URG INTEGER
opt_tcp_options: %empty
| '<' tcp_option_list '>'
| '<' ELLIPSIS '>'
tcp_option_list: tcp_option
| tcp_option_list ',' tcp_option
opt_tcp_fast_open_cookie: %empty
| hex_blob
hex_blob: WORD
| INTEGER
tcp_option: NOP
| EOL
| MSS INTEGER
| WSCALE INTEGER
| SACKOK
| SACK sack_block_list
| MD5 hex_blob
| TIMESTAMP VAL INTEGER ECR INTEGER
| FAST_OPEN opt_tcp_fast_open_cookie
| FAST_OPEN_EXP opt_tcp_fast_open_cookie
sack_block_list: sack_block
| sack_block_list sack_block
sack_block: INTEGER ':' INTEGER
syscall_spec: opt_end_time function_name function_arguments '=' expression opt_errno opt_note
opt_end_time: %empty
| ELLIPSIS time
function_name: WORD
function_arguments: '(' ')'
| '(' expression_list ')'
expression_list: expression
| expression_list ',' expression
expression: ELLIPSIS
| any_int
| WORD
| STRING
| STRING ELLIPSIS
| binary_expression
| array
| inaddr
| in6addr
| sockaddr
| msghdr
| iovec
| pollfd
| linger
| mpls_stack_expression
| cmsg_expr
| scm_timestamping_expr
| sub_expr_list
| sock_extended_err_expr
| '{' gre_header_expression '}'
| epollev
any_int: decimal_integer
| hex_integer
decimal_integer: INTEGER
hex_integer: HEX_INTEGER
binary_expression: expression '|' expression
| WORD '=' expression
array: '[' ']'
| '[' expression_list ']'
inaddr: INET_ADDR '(' STRING ')'
in6addr: INET6_ADDR '(' STRING ')'
sockaddr: '{' SA_FAMILY '=' WORD ',' SIN_PORT '=' _HTONS_ '(' INTEGER ')' ',' SIN_ADDR '=' INET_ADDR '(' STRING ')' '}'
msghdr: '{' MSG_NAME '(' ELLIPSIS ')' '=' ELLIPSIS ',' MSG_IOV '(' decimal_integer ')' '=' array ',' MSG_FLAGS '=' expression opt_cmsg '}'
opt_cmsg: %empty
| ',' MSG_CONTROL '=' array
cmsg_expr: '{' CMSG_LEVEL '=' expression ',' CMSG_TYPE '=' expression ',' CMSG_DATA '=' expression '}'
scm_timestamping_expr: '{' SCM_SEC '=' INTEGER ',' SCM_NSEC '=' INTEGER '}'
sub_expr_list: '{' expression_list '}'
sock_extended_err_expr: '{' EE_ERRNO '=' expression ',' EE_ORIGIN '=' expression ',' EE_TYPE '=' expression ',' EE_CODE '=' expression ',' EE_INFO '=' expression ',' EE_DATA '=' expression '}'
iovec: '{' ELLIPSIS ',' decimal_integer '}'
pollfd: '{' FD '=' expression ',' EVENTS '=' expression opt_revents '}'
epollev: '{' EVENTS '=' expression ',' FD '=' expression '}'
| '{' EVENTS '=' expression ',' PTR '=' expression '}'
| '{' EVENTS '=' expression ',' U32 '=' expression '}'
| '{' EVENTS '=' expression ',' U64 '=' expression '}'
opt_revents: %empty
| ',' REVENTS '=' expression
linger: '{' ONOFF '=' INTEGER ',' LINGER '=' INTEGER '}'
mpls_stack_expression: '{' mpls_stack '}'
opt_errno: %empty
| WORD note
opt_note: %empty
| note
note: '(' word_list ')'
word_list: WORD
| FLAGS
| word_list WORD
command_spec: BACK_QUOTED
code_spec: CODE
opt_cleanup_command: %empty
| cleanup_command
cleanup_command: command_spec